In recent years, small-sized low-price laser light sources have been manufactured. In addition, going with advancement in a MEMS (Micro-Electro-Mechanical Systems) technology, small-sized scanning elements that vibrate rapidly have been developed. Under such a background, a scanning-type laser projector has been developed which draws video by varying the angle of laser light with a MEMS mirror or the like to perform scanning on a projected plane (see, for example, Patent Document 1).
On the other hand, products using lasers are required to guarantee safety, and it is necessary to follow the safety regulations stipulated in the standards such as IEC (International Electrotechnical Commission) 60825 and JIS (Japan Industrial Standard) 6802.
In order to guarantee the safety of lasers, it is prescribed that the lasers are classified in conformity with the standards such as IEC 60825 and JIS 6802 and usage methods depending on classes must be followed. Lasers are classified into seven grades including classes 1, 1M, 2, 2M, 3R, 3B, and 4, and applicable purposes and applicable environments are limited depending on the respective classes. Here, the classification of the seven grades is recited in IEC 60825-1 published in 2007. In the following, a description conforming to this IEC will be given.
IEC 60825-1 stipulates that the safety of scanning-type lasers is evaluated using a repetitive pulse train in which a pulse time width is defined as a time that is required for a laser to pass through an aperture that has a diameter of 7 mm, simulates an eye of humans, and is disposed so as to be apart from a scanning unit by a given distance, and the number of pulses is defined as the number of times the laser passes through this aperture. In addition, IEC 60825-1 stipulates that the safety is to be evaluated under the most severe condition among the following three conditions about the repetitive pulse train. Herein, an example of a calculation conforming to the stipulation of class 2 will be shown. Here, C6 is a correction factor depending on the size of a light source that is stipulated in the standard, t is the pulse time width, N is the number of pulses, and π is the ratio of the circumference of a circle to its diameter.
A first condition is that the exposure from any single pulse within a pulse train shall not exceed the AEL (Accessible Emission Limit) for a single pulse.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                      P          ⁡                      (            a            )                          =                  5          ×                      10                          -              3                                ⁢                                                    C                6                            (                                                7                  ×                                      10                                          -                      3                                                                      2                            )                        2                    ⁢          π          ⁢                      1            t                                              first        ⁢                                  ⁢        condition            
A second condition is that the average power for a pulse train of emission duration shall not exceed the AEL for a single pulse of an emission time width.
                    [                  Formula          ⁢                                          ⁢          2                ]                                                                      P          ⁡                      (            b            )                          =                                            18.025              0.75                        N                    ⁢                                                    C                6                            (                                                7                  ×                                      10                                          -                      3                                                                      2                            )                        2                    ⁢          π          ⁢                      1            t                                              second        ⁢                                  ⁢        condition            
A third condition is that the average exposure for pulses within a pulse train shall not exceed the AEL for a single pulse multiplied by the correction factor C5=N−1/4.
                    [                  Formula          ⁢                                          ⁢          3                ]                                                                      P          ⁡                      (            c            )                          =                  5          ×                      10                          -              3                                ⁢                      C            5                    ×                                                    C                6                            (                                                7                  ×                                      10                                          -                      3                                                                      2                            )                        2                    ⁢          π          ⁢                      1            t                                              third        ⁢                                  ⁢        condition            
As is clear from the comparison with the third condition, the first condition is always laxer than the third condition, and thus it is not necessary to take the first condition into consideration.
By the way, as stipulated in IEC 60825-1, the safety must be taken into consideration under the situation in which the presumable largest exposure is experienced. When reciprocating scanning of a laser is performed, end portions are the positions at which the exposure level is largest. There are two reasons. A first reason is that when the laser is turned around, the laser is continuously irradiated by the period twice that in one-way scanning. A second reason is that particularly when a scanning element is a device using resonance, the scanning speed of the laser is significantly reduced at the end portions.
The upper limit of an output of a laser is determined by those at the end portions where the exposure level is largest, and the brightness of the entire screen is determined by those at the end portions. At an end portion, the number of pulses is reduced by a half and the passage period doubles. Assuming that this is applied to the second condition and the third condition, in the case of, for example, the third condition, the value of P(c) becomes 2−3/4(=(½)−1/4×(½))≈0.6 times smaller than the value obtained in accordance with the above-described formula.